1. Field of the Invention
The present invention relates to an optical coherence tomography apparatus which irradiates an object with low-coherence light as signal light, and acquires a tomographic image of the object, where the tomographic image represents information on fine structures on and under a surface of the object, based on the signal light which is reflected from the surface and subsurface portions of the object.
2. Description of the Related Art
Conventionally, optical coherence tomography apparatuses using low-coherence light are used. In particular, optical coherence tomography apparatuses in which intensities of low-coherence interference light are detected by heterodyne detection are used for obtaining a tomographic image of subretinal fine structures or the like.
In the above optical coherence tomography apparatuses, low-coherence light emitted from a light source such as a superluminescent diode (SLD) is split into signal light and reference light, and the frequency of the reference light is slightly shifted by using a piezo electric element or the like. Next, an object is irradiated with the signal light, and the reference light and reflected light from a predetermined depth in the object are optically multiplexed so as to produce interference light. Then, the intensity of the reflected light is detected by heterodyne detection based on interference light in order to obtain tomographic information. In this detection, information on a deep portion of the object, to which the round trip optical length of the signal light coincides with the optical length of the reference light, is obtained. In addition, when the optical length of the reference light is varied by moving a movable mirror or the like which is arranged in an optical path of the reference light, information on another area of the object located at a different depth can be obtained.
In the above optical coherence tomography apparatuses, it is desirable that the interference between the reference light and the signal light occurs only when the lengths of the optical paths of the reference light and the signal light precisely coincide. However, in practice, the interference between the reference light and the signal light occurs when the difference between the lengths of the optical paths of the reference light and the signal light is equal to or less than the coherence length of the light source, the interference occurs. That is, the resolution in the low-coherence interference is determined by the coherence length of the light source.
Generally, the coherence length is dependent on the type of the light source, the oscillation mode, noise, and the like. Usually, when laser light is used as the above low-coherence light, it is possible to regard the coherence length as being approximately proportional to the pulse width.
For example, when a pulse laser which emits pulsed laser light having a center wavelength of 800 nm and a pulse width of 25 fs (i.e., 25xc3x9710xe2x88x9215 sec) is used, the coherence length is about 14 micrometers. In addition, when an SLD which emits pulsed light having a center wavelength of 800 nm and a spectral width of 20 nm is used, the coherence length is also about 14 micrometers. That is, when these are used as the light sources in the above optical coherence tomography apparatuses, the resolution is about 14 micrometers. Therefore, when the object includes a plurality of layers within a thickness equal to the coherence length, it is not possible to distinguish the respective layers based on the reflected light.
In addition, recently, in the field of clinical medicine, usefulness of the tomographic image of living tissue is widely known. For example, demands for high-resolution tomographic images of living tissue which scatters light more highly than eyeballs are growing. In order to satisfy the above demands, a light source which can emit low-coherence light having a low coherence length and high output power is necessary. However, it is impossible to increase the output power of the SLD. In addition, it is also impossible to reduce the pulse width and the coherence length of the SLD since the pulse width of the SLD is determined by its bandgap.
In order to solve the above problem, for example, B. E. Bouma et al., (xe2x80x9cSelf-phase-modulated Kerr-lens mode-locked Cr:forsterite laser source for optical coherence tomography,xe2x80x9d Optics Letters Vol. 21, Issue 22, pp.1839-1841, November 1996) propose an apparatus which can obtain a high-resolution tomographic image by using low-coherence light having a short pulse width and high output power. The apparatus includes as a light source a KLM (Kerr-lens mode-locked) Ti:sapphire laser which emits an ultrashort-pulsed light having a pulse width of a few femtoseconds. In this apparatus, low-coherence light having a short pulse width and high output power is obtained by using the above ultrashort-pulsed light, and used as signal light and reference light in order to obtain a high-resolution tomographic image.
However, in the above apparatus, the light source including the KLM (Kerr-lens mode-locked) Ti:sapphire laser is bulky, expensive, and uneasy to handle. That is, in practice, the above apparatus using the KLM (Kerr-lens mode-locked) Ti:sapphire laser is not practicable due to its large size, high cost, and poor usability.
The object of the present invention is to provide an optical coherence tomography apparatus which uses a small-sized, inexpensive, and easy-to-handle light source, and can acquire a high-resolution tomographic image by using low-coherence interference.
According to the present invention, there is provided an optical coherence tomography apparatus comprising: a light source which emits low-coherence light; an optical splitting unit which splits the low-coherence light into signal light having a first frequency and first reference light having a second frequency; a frequency shifting unit which shifts at least one of the first and second frequencies so that a predetermined frequency difference is produced between the first and second frequencies; an irradiating unit which irradiates an object with the signal light; an optical multiplexing unit which optically multiplexes the reference light and a portion of the signal light which is reflected from a predetermined depth in the object, so as to produce interference light; an image detection unit which detects an optical intensity of the interference light, and obtains a tomographic image of the object based on the optical intensity. The light source comprises a pulsed light source unit which emits pulsed light having a third frequency and a pulse width, and an optical-waveguide structure is made of a material having a normal dispersion characteristic with respect to light which has the third frequency, and includes a structure which realizes an anomalous dispersion characteristic with respect to light which has the third frequency, so as to reduce the pulse width of the pulsed light.
Thus, in the optical coherence tomography apparatus according to the present invention, the pulsed light which has the reduced pulse width is emitted as low-coherence light from the light source.
Specifically, the above predetermined frequency difference is such that a beat signal having a frequency which is equal to the predetermined frequency difference is included in the interference light. The above intensity of the interference light is the intensity of the beat signal. For example, the image detection unit detects the intensity of the beat signal by the heterodyne interferometry or the like.
Generally, the reciprocal of the pulse width of the pulsed laser light emitted from a pulse laser is approximately proportional to the spectral width of the pulsed laser light. That is, when the pulse width is reduced, the spectral width increases, and therefore the coherence length is reduced. Therefore, when the pulse width of the low-coherence light emitted from the light source is reduced, the resolution of the tomographic image can be increased corresponding to the reduction in the coherence length.
In the optical coherence tomography apparatus according to the present invention, the pulse width of the pulsed laser light emitted from the pulsed light source is reduced by the optical-waveguide structure, and the optical-waveguide structure is made of a material having a normal dispersion characteristic with respect to light which has the frequency of the pulsed laser light, and includes a structure which realizes an anomalous dispersion characteristic with respect to the light which has the frequency of the pulsed laser light. That is, low-coherence light having a short coherence length can be obtained by the provision of a small-sized, inexpensive, easy-to-handle light source. In other words, a bulky, expensive, uneasy-to-handle light source, which is required in the conventional optical coherence tomography apparatus, is unnecessary. Thus, the resolution in the low-coherence interference can be improved.
Preferably, the optical coherence tomography apparatus according to the present invention may also have one or any possible combination of the following additional features (i) to (xviii).
(i) The optical-waveguide structure may be a transparent-type finely-structured optical waveguide, a reflection-type Bragg grating, or the like.
(ii) The optical-waveguide structure may comprise at least one Bragg grating formed with a plurality of light-reflecting portions arranged in a longitudinal direction of the optical-waveguide structure at a pitch which varies progressively.
The plurality of light-reflecting portions can be formed by any method. For example, the plurality of light-reflecting portions can be realized by periodically forming projections and depressions in an optical waveguide by etching, or forming cladding having a periodic variation, or periodically forming high-refractive-index portions.
When the optical-waveguide structure is realized by at least one Bragg grating formed with a plurality of light-reflecting portions arranged in a longitudinal direction of the optical-waveguide structure at a pitch which varies progressively, the pitch (or period) of the Bragg grating can be arranged corresponding to the frequency and the pulse width of the pulsed (laser) light emitted from the pulsed light source (e.g., a pulse laser), so that the Bragg grating can efficiently reduce the pulse width of the pulsed (laser) light. In addition, since it is easy to arrange the Bragg grating in the optical path of the pulsed laser light, the light source can be easily manufactured. Even when the pulse width cannot be sufficiently reduced to a desired amount by a Bragg grating, the pulse width can be reduced to the desired amount by arranging a plurality of Bragg gratings in a plurality of stages.
(iii) In the optical coherence tomography apparatus having the feature (ii), the at least one Bragg grating may be at least one frequency-modulation type Bragg grating formed with a plurality of high-refractive-index portions arranged in the longitudinal direction of the optical-waveguide structure at the above-mentioned pitch. In this case, it is easy to form a Bragg grating having a desirable pitch. Therefore, the pulse width can be reduced with high accuracy.
The above frequency-modulation type Bragg grating may be a planar-waveguide type Bragg grating, a fiber Bragg grating, or the like. That is, the above frequency-modulation type Bragg grating may be any Bragg grating which is produced by forming high-refractive-index portions in an optical waveguide at a pitch which varies progressively. Since the fiber grating is easy to place in narrow space, the use of the fiber grating contributes to downsizing of the entire apparatus.
(iv) In the optical coherence tomography apparatus having the feature (ii), the at least one Bragg grating may be at least one linear Bragg grating in which the pitch varies linearly. Since the linear Bragg grating can be easily formed at low cost. Therefore, the cost of the light source can be further reduced.
(v) In the optical coherence tomography apparatus having the feature (ii), the at least one Bragg grating may be at least one chirped fiber Bragg grating.
(vi) The pulsed light source may comprise a fiber laser doped with a rare-earth ion. In this case, it is possible to obtain pulsed laser light having a narrow pulse width and high output power in a desired wavelength band.
(vii) In the optical coherence tomography apparatus having the feature (vi), the pulsed light source may further comprise a second harmonic generator. In this case, it is possible to obtain pulsed laser light in a wavelength band which cannot be obtained from the rare-earth-ion-doped fiber laser per se.
(viii) In the optical coherence tomography apparatus having the feature (vi), the rare-earth ion may be erbium. In this case, pulsed laser light having a narrow pulse width and high output power can be obtained at low cost.
(ix) In the optical coherence tomography apparatus having the feature (vi), the rare-earth ion may be ytterbium. In this case, pulsed laser light having a narrow pulse width and high output power can be obtained at low cost.
(x) The pulsed light source may comprise a titanium sapphire laser. The oscillation wavelength of the titanium sapphire laser is tunable. Therefore, when the titanium sapphire laser is used as the pulsed light source, users can select a desired wavelength band.
(xi) The pulse width of the pulsed light may be in a range from 10 fs to 1 ps. In this case, the pulse width can be efficiently reduced to a desired amount.
(xii) The optical coherence tomography apparatus according to the present invention may further comprise an optical amplifying unit which optically amplifies the reflection light (i.e., the above-mentioned portion of the signal light which is reflected from a predetermined depth in the object) before the reflection light is optically multiplexed with the reference light. It is easy to arrange an optical amplifying unit in the optical path of the reflection light.
(xiii) In the optical coherence tomography apparatus having the feature (xii), the optical amplifying unit may be an optical amplifier comprising an optical waveguide.
(xiv) In the optical coherence tomography apparatus having the feature (xiii), the optical amplifier may be a semiconductor optical amplifier, a Raman amplifier using the stimulated Raman effect, or an optical fiber amplifier. When the optical fiber amplifier is used, it is possible to increase the length of the optical fiber to a length which realizes a desired gain without substantially increasing the size of the optical fiber amplifier, since the optical fiber can be wound for compact placement. Therefore, the reflection light can be amplified with a great gain by using a small-sized optical fiber amplifier unit. Further, since optical fiber amplifiers have low-noise characteristics, it is possible to accurately amplify very weak reflection light.
(xv) In the optical coherence tomography apparatus having the feature (xiv), the optical fiber amplifier may comprise an optical fiber doped with at least one ion from among transition-metal ions, rare-earth ions, and complex ions. In this case, the optical fiber amplifier can achieve a great gain in a desired wavelength band which reflection light belongs to.
(xvi) In the optical coherence tomography apparatus having the feature (xv), the optical fiber may be doped with at least one ion from among transition-metal ions Ti4+, Cr3+, Mn4+, Mn2+, and Fe3+, rare-earth ions Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+, and complex ions WO42xe2x88x92, MoO42xe2x88x92, VO43+, Pt(CN)42xe2x88x92, and WO66xe2x88x92. In addition, since optical fibers can be easily doped with each of these ions, the manufacturing cost of the optical fiber amplifier can be reduced.
(xvii) In the optical coherence tomography apparatus having the feature (xiv), the optical fiber amplifier may comprise a dye-doped optical fiber. In this case, the optical fiber amplifier can achieve a great gain in a desired wavelength band which reflection light belongs to.
(xviii) The object may be a portion of living tissue, and the low-coherence light may have a wavelength in a range from 600 nm to 1,700 nm. In this case, the signal light exhibits desirable transmitting and scattering characteristics in the living tissue, and therefore a desirable tomographic image can be obtained.